Delayed Coking is a well proven and commercialized process for converting residues into lower molecular weight petroleum fractions suitable for treating or further conversion in other refining processes and production of a solid residue byproduct (coke) that contains the majority of the contaminants in residues that are detrimental for processing in other refinery processes. Some of the contaminants in residues do end up in the delayed coking lighter products especially the Heavy Coker Gas Oil (HCGO).
Delayed coking processes have been used in the prior art to thermally decompose heavy liquid hydrocarbons into gases, liquid streams of various boiling ranges, and coke. The delayed coking process involves heating hydrocarbon liquids in a coking furnace and transferring the heated liquids to a coking drum where the liquids decompose into coke and volatile components.
In order to practically use the delayed coking process, a coker fractionation system is needed along with the coking furnace and coking drums. The coker fractionating system separates the volatile components generated in the coking drum into various hydrocarbon streams.
In the basic delayed coking process, a liquid hydrocarbon feedstock is initially added to the bottom of a coker fractionator column where it mixes with the column bottoms liquid which is referred to as “natural recycle material.” This mixture of feedstock and natural recycle material is taken from the fractionator column bottom and then pumped through furnace tubes of the coking furnace where it is heated to about 1000° F. The heated stream is then transferred to the coking drum where the temperature and pressure are maintained at coking conditions such that the stream decomposes into coke and volatile components. The volatile components, called “coke drum vapors”, are then returned to the coker fractionating system for separation into various components. When the coke drum becomes full of solid coke, the heated stream from the coker furnace is diverted to another coke drum and the full coke drum is cooled and emptied.
The coker fractionating system used in the delayed coking process generally includes a fractionator column which includes a reservoir for the heavy recycle material and feedstock mixture at the bottom of the column. Above the reservoir is a flash zone, an open area within the column, into which the coke drum vapors are introduced. The heaviest components of the coke drum vapors are condensed in the flash zone and the remaining vapors are fractionated by multiple trays above the flash zone. At the top of the coker fractionator column is a vapor reflux system in which at least a portion of the overhead vapor stream being discharged from the column is condensed and returned to the top fractionator tray. The remainder of the condensed overhead vapor stream is withdrawn as an unstabilized naphtha product.
Traditionally, two liquid streams are removed from the coker fractionating system at different points in the fractionating column. A light coker gas oil stream is removed from a tray near the top of the fractionator to provide one end product of the system. This is known as the light coker gas oil draw. The second stream is a heavy coker gas oil stream removed near the bottom fractionation tray to provide a second end product of the system. This is known as the heavy coker gas oil draw.
Generally, a portion of this second stream is returned to the column as part of a pump-around system. Pump-around systems are generally used to recover thermal energy from the fractionator column and include a pump and a heat exchanger to provide heat to another process stream or to generate steam. When the pump-around system is connected to the heavy coker gas oil draw, thermal energy is removed from the lower part of the fractionation system. The removal of heat at this point in the column reduces fractionation efficiency and results in a heavy coker gas oil product stream which contains light end hydrocarbons. These light end hydrocarbons are removed by further processing to meet the heavy coker gas oil product's downstream processing specification requirements. Typically, this is done by providing an additional steam stripping system which includes a stripping column, multiple product pumps, and a heat exchanger for recovering heat from the stripping column.
Maximizing liquid yields in delayed coking is usually desirable for most applications, especially when making fuel grade coke where the coke's value is relatively low compared to the distillable products from the coking process. When maximizing the liquid yields, typically the HCGO yield and its end boiling point are maximized within the capabilities of the delayed coking process. Accordingly, when maximizing the HCGO yield and end boiling point, the HCGO's contaminants such as Sulfur, Nitrogen, multi-ring aromatics, and asphaltenes increase significantly (see FIG. 1 and FIG. 2). FIG. 1 shows a hydrocracking process using a combined feed. The feed rate to the hydrocracking process increased with HCGO end point and this raises conversion to valuable distillate range products. The maximum HCGO end-point is determined by contaminant levels in the blended feed, the quantity of C7 insolubles, which is critical and the need to assess the impact on the hydrocracking unit. FIG. 2 shows the properties of HCGO as the end-point increases. At a higher HCGO end-point, the amount of metals, Conradson carbon and asphaltenes increase rapidly, the hydrocracking unit capacity and cost increases, and the delayed coking unit cost decreases due to the lower recycle. These contaminants, especially multi-ring aromatics and asphaltenes, can pose a problem in the downstream vacuum gas oil conversion units, such as hydrocrackers. The delayed coker operation may then be constrained by limitations imposed by downstream processing units because of the negative impacts of the highest end point components of HCGO on downstream vacuum gas oil (VGO) conversion processes, especially hydrocracking's catalyst life. Table 1 shows the impact of increasing the HCGO end point on hydrocracking unit operation. Contaminant levels at the highest HCGO end point cause excessive catalyst deactivation.
TABLE 1HCGO End Point, ° C.Base+21+41+54Stage 1 Liquid FeedrateBase+4%+7%+8%Stage 2 Liquid FeedrateBase+4%+7%+8%PressureBaseBaseBaseBaseMake-up Gas RateBase+7%+13% +17% Recycle Gas RateBase+4%+7%+8%Catalyst VolumeR-1 (First Stage)Base+6%+12% +25% R-2 (First Stage)Base+8%+14% +23% R-3 (Second Stage)Base+4%+7%+8%
If these contaminants are removed, the downstream processing costs will be significantly reduced and the liquid yields from the combined delayed coker and the downstream VGO Hydrocracking or FCC processes will be maximized. Maximizing the end boiling point of HCGO directionally will maximize the upgrading margins for most transportation fuels applications. An example of the benefits is shown in FIG. 3. As shown in FIG. 3, incrementally raising the HCGO end point to the highest practicable level, increases the product value of the hydrocracked products by almost 100 million dollars per year. In return for an incremental investment that is relatively low, coker cost reduction partially offsets hydrocracking unit cost increases. Thus, there is a strong economic incentive to maximize the HCGO end point.
Thus, it would be advantageous to have a delayed coker design that can maximize its HCGO yield while producing a HCGO suitable for VGO Hydrocracking, which would have both liquid yield and economic benefits.
Typical delayed coking units have configurations such as shown in FIG. 4. Feed typically enters the lower zone of the fractionator where it is mixed with any recycle streams such as HCGO that is condensed from the cooling of the coke drum vapors in the fractionator. This also provides a surge capacity resulting in a steady feed rate to the coke drums and with consistent feed quality. The fractionator bottom stream is then heated and sent to the coke drums where majority of the thermal cracking reactions occur.
In an alternative form of the delayed coker, typically referred to a zero recycle coking, feed is sent directly to the process heater and a heavier HCGO product (HHCGO) is drawn from the bottom of the fractionator (FIG. 5).
Table 2 shows the typical yields when processing a medium sour vacuum residue. Zero recycle coking typically increases HCGO liquid yield by 3-4 volume %. Coke is reduced by 1-2 weight %.
TABLE 2LowZeroRecycleRecycleIncrementalPressure, psig1515Recycle ratio1.051.00−0.05DRYGAS, wt %3.803.79−0.01LPG, vol %6.776.58−0.19Naphtha, vol %13.8612.91−0.95LCGO, vol %25.8624.11−1.75HCGO, vol %34.3837.563.18C5+ liquids, vol %74.0174.580.57Coke, wt %27.6726.53−1.14
Table 2 shows the properties of HCGO with conventional low recycle coking and zero recycle coking and how the HCGO properties deteriorate as the HCGO end point is increased and maximized in the case of zero recycle coking. The deterioration in properties results in most delayed coking process designs for transportation fuel applications limiting the end point of the HCGO to about 1065° F. which is obtainable with low recycle and pressure coking, particularly when HCGO is sent to a VGO hydrocracking process.